This invention relates to superjunction semiconductor devices and more specifically relates to a novel process for forming a uniform implant and diffusion into the side walls of spaced cell elements of a superjunction device.
Superjunction devices are well known and are described, for example, in U.S. Pat. Nos. 4,754,310 to Coe; 5,216,275 to Chen; and 3,925,803 to Sony.
Although the so called superjunction has brought a new concept to power MOSFET devices, the present processes to realize the design concept are difficult and complex. In one known process, a relatively shallow Nxe2x88x92 epitaxial layer is formed atop an N+ substrate, and spaced P diffusions which form portions of P columns are diffused in the layer. A series of such N layers are grown atop one another, with corresponding P diffusions stacked atop one another until the desired P column height is reached. A typical process could be realized by using multi-implants and epitaxial growths as shown in FIG. 1.
Such a fabrication process is not only complex and expensive, but also degrades the electrical characteristics because of the non-uniform doping profile formed by multi-implants and epitaxial growths. Other processes have been proposed in which trenches are etched in an N body and then filled in with a further layer of P material. A process of this type is shown in copending application Ser. No. 09/732,401, filed Dec. 7, 1900 entitled HIGH VOLTAGE VERTICAL CONDUCTION SUPERJUNCTION SEMICONDUCTOR DEVICE in the names of Daniel M. Kinzer and Srikant Sridevan (IR-1756). These processes are bulky and complex, particularly when spaced cells or pylons of one concentration type is to be formed in a body of a different conductivity type.
Still another process is known in which parallel trenches are formed in an Nxe2x88x92 epitaxial layer and in which the walls of the trenches are simply diffused with a P type dopant of sufficient concentration and depth to be in charge balance with the surrounding N+ epitaxial body. In such devices, it is difficult to obtain uniform P type concentration along the length of the walls of the trench, particularly for a deep trench.
In this invention, an angle implant is provided for directly doping the sidewalls of deep cells in a uniform and well controlled manner. The basic concept of the invention is that after the deep trench etching, an angle implant is used to dope the trench sidewall. The implant angle can be easily adjusted by the implant machine to assure that doping ions reach the bottom of the trench. The selected angle value depends on the depth and width of the trench. By further rotating the implant sample wafers, either intermittently or continuously, the trench sidewalls are uniformly doped along their full length. The cross-section oft he trenches can have different forms, such as hexagon, stripe, circle, or rectangle and the like. The doping distribution, junction depth and surface concentration of the trenches are determined by implant energy, dose, species and as annealing conditions. The main advantages of the invention are as follows: (1) there is only a single implant and no epitaxial growth is needed for a trench; (2) the trenched sidewall can be easily and uniformly doped in one operation; (3), the cost, in comparison with the conventional trench formation, is greatly reduced; (4) ultra shallow junctions can be formed and controlled, which is essential for reducing transistor cell size; (5) the generated uniform doping profile formed by direction implantation is critical for establishment of an ideal regular rectangle shape for the characteristic curve of electrical field along the trench depth, which is a basic requirement for a superjunction device.